The emission of toxic metals has become an environmental issue of increasing concern because of the dangers posed to human health. For instance, coal-fired power plants and medical waste incineration are major sources of human activity related to toxic metal emission into the atmosphere. However, emission control regulations have not been rigorously implemented for coal-fired power plants. A major reason is a lack of effective control technologies available at a reasonable cost.
A technology currently in use for controlling mercury emissions from coal-fired power plants is activated carbon injection (ACI). The ACI process involves injecting activated carbon powder into a flue gas stream and using a fabric filter or electrostatic precipitator to collect the activated carbon powder that has sorbed mercury. ACI technologies generally require a high C:Hg ratio to achieve the desired mercury removal level, which results in a high cost for sorbent material. The high C:Hg ratio indicates that ACI does not utilize the mercury sorption capacity of carbon powder efficiently.
An activated carbon packed bed can reach high mercury removal levels with more effective utilization of sorbent material. On the other hand, a typical powder or pellet packed bed has a very high pressure drop, which significantly reduces energy efficiency. Further, these fixed beds are generally an interruptive technology because they require frequent replacement of the sorbent material.
Flow-through substrates such as honeycombs may be used to achieve high removal levels of toxic metals from gas streams with lower pressure drop. Some gas streams, however, may contain agents that would inhibit the sorption of toxic metals on sorbents such as activated carbon honeycombs and other sorbent compositions and structures.